The present invention relates to a control apparatus for a fluorescent lamp having an oscillation circuit for lighting control of a fluorescent lamp in accordance with the output frequency of the oscillation circuit.
The fluorescent lamp hitherto in use has filaments disposed on its both ends and has a load circuit formed of the fluorescent lamp with the filaments, a capacitor connected between the filaments, and a choke coil connected in series with the filament. When a voltage of a predetermined frequency according to the output frequency of the oscillation circuit is applied to the load circuit the filaments are preheated so that a discharge is passed between the filaments and light is emitted.
FIG. 2 shows the relationship between the output frequency f of the oscillation circuit and the tube current I flowing through the load circuit (f-I characteristic). When the fluorescent lamp is normally lighted, the relationship between the output frequency f and the tube current I has a virtually linear characteristic as indicated by a in FIG. 2, namely the tube current I decrease as the output frequency f increases. However, when no discharge takes place between the filaments, the capacitor, choke coil, and the resistance of the filaments come to be connected in series, and, hence, the f-I characteristic indicated by b in FIG. 2, is such that the current reaches a maximum at the resonant frequency f0.
The oscillation circuit, in the normal state, operates to light the fluorescent lamp at a fundamental frequency f1 (period: T1) of the value which is different from the resonant frequency f0. However, when the fluorescent light is extinguished on the lamp is exchanged, in order to pass a large tube current through the load circuit so that the fluorescent lamp is automatically lit again, it is arranged in the modulation circuit such that the output frequency f is modulated to a frequency f2 closer to the resonant frequency f0 than the fundamental frequency f1 at intervals of a predetermined period T3 (for example 4 msec) as shown in FIG. 3. By execution of such modulation, when the fluorescent lamp is in its lighted state, the tube current I becomes smaller during the execution of the modulation as shown in FIG. 4. However, when the lamp is in its extinguished state, the tube current I becomes greater as shown in FIG. 5. By the greater tube current I provided at this time, the filaments are preheated and a discharge is caused to take place, so that the fluorescent lamp automatically starts to emit light again.
However, since the preheating current flows through the filament as described above, the filament undergoes aged deterioration, i.e., it gradually sublimes and becomes thinner, and it eventually leads to defective lighting of the fluorescent lamp. At such an end of life of the fluorescent lamp, a current still flows through the load circuit because the filament is not yet broken. Nevertheless, the lamp becomes unable to come on again even if the above described modulation is performed, or, even when it is turned on, it immediately goes out. This produces a large tube current flowing through the fluorescent lamp every time the modulation is performed. If such a condition lasts long, an abnormal temperature rise is caused in the choke coil or an unpleasant state for lighting apparatus occurs such that the fluorescent lamp at the end of life repeatedly goes on and out. Such a problem becomes more severe when the fluorescent lamp is being dimmed.
Especially when a large number of fluorescent lamps are being lit, if one of the fluorescent lamps reaches its end of life, all the fluorescent lamps must be put out for the protection of the circuit, or if it is desired to put out only the fluorescent lamp reaching its end of life, a problem arises in that a detection circuit must be provided for each of the fluorescent lamps.
There has also been such a problem that, when the power supply voltage becomes low for some reason or other and the voltage applied to the oscillation circuit is thereby lowered, operation of the oscillating circuit becomes unstable and an abnormal oscillating operation is made.